The present invention relates in a general way to machines for grinding ophthalmic glasses, and more particularly to a device for regulating the grinding weight for such machines.
The invention concerns more particularly a device for regulating the grinding weight in a machine for grinding ophthalmic glasses of the type comprising a carriage slidably and pivotally mounted on a shaft, and an arm which is connected to the shaft and bears against and is slidable along a rail parallel to said shaft, the arm bearing against the rail through an element for actuating a sensor mounted on the arm, a roller being interposed between the rail and the actuating element and being mounted on the latter. Such a device was described and represented in the French patent application No. 2,639,276 filed on Nov. 22, 1988 in the name of the applicant.
As described in said patent application and as known per se, a conventional machine for grinding ophthalmic glasses comprises a generally U-shaped carriage which is slidably and pivotally mounted on a first shaft by an intermediate part of the carriage, a second shaft parallel to said first shaft and carrying a group of grinding wheels driven in rotation by a motor, wings of the carriage carrying a third shaft which is parallel to the first and second shafts and is in two parts between which a blank to be ground is maintained, and an arm connected to the carriage and carrying a tracer member against which bears a template mounted on an extension of said third shaft.
In operation, as the blank to be ground is gripped between the two shaft parts, between the wings of the carriage, it is brought into contact with a rotating grinding wheel while being driven in rotation about itself until the blank has been reduced to the dimensions of the template.
This operation for peripherally grinding the blank requires applying the latter against the grinding wheel with a certain force which, in the conventional machines, was produced merely by the weight of the carriage, it being possible if necessary to correct this weight by means of a spring. However, it was found necessary to vary this force in order to regulate the depth of the grinding pass as a function of the nature and/or the thickness of the glass to be ground.
According to a first solution prior to said French patent application No. 2,639,276, a counterweight is employed mounted on the carriage and movable in a direction perpendicular to the shafts in one direction or the other, so as to increase or decrease the force with which the blank is applied against the grinding wheel. In a variant, it is also possible to employ a spring.
In a second solution, described in French patent No. 2,481,635 of the applicant, the arm carrying the tracer member bears against and slides along a rail parallel to the aforementioned shafts, and the tracer member is itself mounted on a jack which bears against a compression spring and is connected to a displacment sensor connected to an electronic device controlling the jack.
These different systems whereby it is possible to control the force with which the blank is applied against the grinding wheel were not fully satisfactory. Indeed, in the case where a counterweight or a spring is used, the regulation is effected manually and this necessarily results in a certain imprecision which cannot be ignored.
In the case of the device disclosed in said French patent No. 2,481,635, it is found that wearing of the abutment of the jack against the compression spring occurs, which results in play producing mechanical deviations.